The invention relates to novel specially substituted urethane acrylates based on tris(p-isocyanatophenyl)thiophosphate having a high refractive index and reduced double bond density and to a process for the preparation thereof. In addition, the invention relates to a photopolymer formulation comprising the urethane acrylates and the use of the photopolymer formulation.
For the uses of photopolymer formulations in the fields of use described below, the refractive index modulation Δn produced by the holographic exposure in the photopolymer plays a decisive role. In the holographic exposure, the interference field comprising signal and reference light beam (in the simplest case, the two plane waves) is formed by the local photopolymerization of, for example, highly refracting acrylates at sites of high intensity in the interference field in a refractive index grating. The refractive index grating in the photopolymer (the hologram) contains all information of the signal light beam. By illuminating the hologram only with the reference light beam, the signal can then be reconstructed again. The strength of the signal thus reconstructed in relation to the strength of the incident reference light is referred to as Diffraction Efficiency, or DE below.
In the simplest case of a hologram which forms from the superposition of two plane waves, the DE is obtained from the quotient of the intensity of the light diffracted in the reconstruction and the sum of the intensities of incident reference light and diffracted light. The higher the DE, the more efficient is a hologram with respect to the necessary quantity of reference light, which is necessary for making the signal visible with a fixed brightness.
Highly refracting acrylates are capable of producing refractive index gratings with high amplitude between regions with lowest refractive index and regions with highest refractive index and hence permitting holograms having high DE and high Δn in photopolymer formulations. It should be noted that the DE depends on the product of Δn and the photopolymer layer thickness d. The greater the product, the greater is the possible DE (for reflection holograms).
The width of the angular range in which the hologram is visible (reconstructed), for example in the case of monochromatic illumination, depends only on the layer thickness d. On illumination of the hologram with, for example, white light, the width of the spectral range which can contribute to the reconstruction of the hologram likewise depends only on the layer thickness d. It is true that the smaller d, the greater the respective acceptance widths.
If it is intended to produce light and easily visible holograms, a high Δn and a low thickness d are desirable, in particular so that DE is as large as possible. This means that the higher Δn, the more latitude there is for producing light holograms by adaptation of d and without loss of DE. The optimization of Δn in the optimization of photopolymer formulations is therefore of outstanding importance.
WO 2008/125229 A1 describes photopolymer formulations which are suitable for producing holograms. These comprise polyurethane-based matrix polymers, acrylate-based writing monomers and photoinitiators. In the cured state, the writing monomers and the photoinitiators are embedded with spatial distribution in the polyurethane matrix. The WO document likewise discloses the addition of dibutyl phthalate, a classical plasticizer for industrial plastics, to the photopolymer formulation.
The holograms obtainable with the aid of the known photopolymer formulation have an insufficient trueness of colour or trueness of angle, i.e. the colours or angles at which the holograms were recorded are not reproduced in the desired manner in the reconstruction of the holograms, i.e. only insufficiently close to the recording conditions of the hologram.
In the following section “Measurement of the holographic properties DE and Δn of the holographic media by means of two-beam interference in reflection arrangement”, it is shown that, owing to resulting shrinkage and owing to changes in the mean refractive index in the photopolymerization of the writing monomers, the angle difference between recording and reconstruction angles must be corrected. The change in the refractive index results mainly from the change of the density of the material on transformation from the monomer to the polymer. In visual holograms, shrinkage leads to insufficient trueness of colour, i.e. the original wavelength which was used for the holographic recording of the object is shifted to shorter wavelength in the reconstruction with the same geometry. In holographic optical elements, in the simplest case of a grating, shrinkage also leads to insufficient trueness of angle.
Particularly if the identical wavelength is used for the holographic recording of simple reflection gratings, the peak shift (α0′-α0) between the reconstruction angle with maximum diffraction efficiency α0′ and the recording angle α0 should be taken into account. In order nevertheless to obtain high trueness of colour and high trueness of angle between recorded and reproduced hologram, a certain effort must therefore be made for angle or wavelength correction or other optical measures must be carried out for compensation.